Poly (lactic acid) (PLA) is biodegradable and biocompatible aliphatic polyester which has useful properties and is derived from renewable feedstocks. Poly (lactic acid) has been reported to have useful applications and is a replacement for petroleum derived polymers such as polyolefins, polyesters and polystyrene (Auras, R.; Lim, L-T; Selke, S. E. M.; Tsuji, H. eds., Poly (Lactic acid): Synthesis, structures, properties, processing and applications; Wiley: N.J., USA., 2010).
PLA is generally prepared by three methods. One, by the Ring Opening Polymerization (ROP) of a cyclic lactide, derived from the dimerization of lactic acid. ROP is the preferred method because it results in high yields of the polymer with high molecular weights. However, ROP requires the preparation of the cyclic dimer of the lactic acid in a separate step followed by purification by melt crystallization. These steps add to the complexity as well as to the cost of producing PLA by ROP. The second method is by the dehydration-polycondensation of aqueous lactic acid. Lactic acid is normally obtained as 80-90% aqueous solution from a fermentor. Direct polycondensation of the aqueous solution requires first removal of water by using an organic solvent as an entrainer for azeotropic distillation followed by polycondensation of the dehydrated lactic acid (water content less than 3 ppm) in presence of a catalyst (Ajioka, M.; Enomoto, K.; Suzuki, K.; Yamaguchi, A.; Bulletin of Chemical Society Japan, 68, 2125-2131, 1995; Moon, S-L.; Lee, C. W.; Miyamoto, M.; Kimura, Y.; Journal of Polymer Science, Polymer Chemistry, 38, 1673-1679, 2000). This process is characterized by very slow rates of polymerization, formation of cyclic oligomers along with the linear polymer, poor color of the final product and difficulties in attaining adequate molecular weights. The third method is by the polyesterification of linear oligomers of PLA of low molecular weight using an esterification catalyst under anhydrous conditions. Though this method yields polymers with reasonable molecular weights, it suffers from the disadvantage that the esterification catalyst is needed in stoichiometric quantities (Shyamroy, S.; Garnaik, B.; Sivaram, S.; Polymer Bulletin, 72(3), 12-, 2014).
Consequently, the latter two methods for producing PLA are not used in commercial practice.
The difficulties in polymerizing aqueous lactic acid arise out of many factors. Lactic acid molecule has two functional groups, i.e. a hydroxyl and a carboxylic acid group which can undergo both intermolecular and intramolecular esterification reactions which are catalyzed by the acid itself. The first step is intermolecular esterification of one unit of lactic acid with a second unit of another lactic acid to form a dimer called lactoyl lactic acid. Further condensation of lactoyl lactic acid proceeds with the removal of water to higher oligomers. The formation of cyclic dimer, lactide, also occurs during this reaction by intramolecular esterification of lactoyl lactic acid or by breakdown of higher oligomers. Both these reactions are equilibrium controlled reactions and proceed with the removal of water. Removal of water is difficult as the polymer molecular weight increases. This causes the equilibrium to reverse, leading to low molecular weight polymers along with substantial amount of cyclic oligomers.
One method to obviate this problem is to provide conditions of polymerization where, instead of water, one can remove more volatile alcohols. In fact such an approach is the basis for the large volume manufacture of aromatic polyesters from aromatic diesters and aliphatic diols, e.g., polyethylene terephthalate which is accompanied by the removal of methanol. Alkyl lactates could also undergo such a reaction. It is interesting to note that during the purification of lactic acid from the fermentation process methyl lactate is an intermediate. Thus, they are available readily.
However, alkyl lactates cannot be polymerized, because they possess low boiling points and they distill before the actual temperature needed for polymerization is reached. Therefore, such reactions are accompanied by poor yields (Marques, D. S; Gil, M. H.; Cristina, M. S.; Baptista, M. S. G.; Journal of Applied Polymer Science, 125(S2), E283-E289, 2012).
Another method to obviate the drawbacks of the aforementioned processes is to provide a self polymerizable monomer containing a hydroxyl group and an alkyl ester group, whose boiling point is substantially higher than the temperatures normally employed for polycondensation. Furthermore, such a monomer must be capable of being synthesized in high chemical purity and be obtained in anhydrous conditions.
One such monomer is an alkyl ester of lactoyl lactic acid, namely, alkyl lactylactate. U.S. Pat. No. 2,371,281 discloses the process for the synthesis of alkyl lactoyl lactate comprising the steps of alcoholysis of lactide in benzene using sulfuric acid as catalyst at 70-75° C. The resulting alkyl lactyl lactate had high boiling points and could be purified by vacuum distillation. Canadian Pat. No. 2656697 discloses a process for making an alkyl lactyl lactate by the reaction of cyclic lactide with a hydroxyl containing compound in presence of acid catalyst in the temperature range of 20° C. to 70° C. Other catalysts have also been reported for the alcoholysis of cyclic lactides to produce alkyl lactyl lactate. For example a magnesium catalyst was reported to yield high yields of ethyl and methyl lactyl lactates (Grala, A.; Ejfler, J.; Jerzykiewicz, L. B.; Sobota, P.; Dalton Transactions, 40, pp 4042-4044, 2011). Metal amides have been used as catalysts for the reaction of methanol with cyclic lactide to yield methyl lactyl lactate (Phomphrai, K.; Pracha, S.; Phonjantheuk, P; Pohmakotr, M.; Dalton Transactions, pp 3048-3050, 2008).
However all the above methods suffer from the disadvantage that the preparation of an alkyl lactyl lactate requires the cyclic lactide as the starting material. The cyclic lactide in high purity has to be made from oligomers of lactic acid followed by melt purification involving multiple steps.
Therefore, one of the objectives of the present invention is to prepare alkyl esters of lactyl lactic acid from a simple esterification of lactic acid with alkyl lactate. Another objective of the present invention is to examine the possibility of transesterification—polycondensation of alkyl lactyl lactate to poly lactic acid. Alkyl lactyl lactates are expected to have high boiling points, low vapour pressures, can be easily purified by distillation, are hydrophobic and possess no acid functionality.
Consequently, one of the objectives of this invention is to explore an efficient method for the synthesis of an alkyl lactyl lactate by the direct esterification reaction of a alkyl ester of lactic acid with lactic acid using a suitable esterification catalyst/reagent. Another objective of the present invention is to avoid the use of cyclic lactide for the preparation of the alkyl lactyl lactate. Another objective of the present invention is to examine the transesterifcation—polycondensation of the ester to produce poly lactic acid.